Steel having good wear resistance

ABSTRACT

A steel having good wear resistance which contains 0.55-1.10% of C, up to 2.0% of Si, up to 2.0% of Mn, 12.0-25.0% of Cr and particularly suitable as a material for piston rings or rocker arms. Further addition of 0.05-1.10% of Al to the above steel gives a steel improved in properties for spheroidization of carbides and uniformization of carbide particle size, leading to further enhancement of wear resistance. If required 0.20-2.0% of Ni may be added to the steels, whereby corrosion resistance, toughness and hardenability are improved. An addition of at least one of 0.2-3.0% of Mo, 0.1-1.5% of V and 0.05-0.70% of Nb to the steels improves high-temperature strength and surface hardness. The steel is surface treated by gas nitriding of the like in a surface area for sliding contact with an opponent member.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to a steel having good wear resistance,and more particularly to a steel and wrought steel having good wearresistance suitable for use as a material for piston rings and rockerarms of internal combustion engines, and pinion shafts for differentialgears, all for parts of automobiles, as well as good fitting strengthand fatigue strength.

2. Description of the Prior Art:

Piston rings used for internal combustion engines consist of compressionrings for maintaining the gas-tightness of combustion chambers and oilscraper rings for conditioning lubricating oil films on the wallsurfaces of cylinders or cylinder liners of the engines. Of the pistonrings, the compression rings are loosely fitted directly below a pistonhead and heavily affected by a combustion gas. Therefore, thecompression rings are required to be resistant to wear (abrasive wearunder the action of carbon soot and corrosive wear under the action ofcorrosive combustion products), scuffing, heat, etc.

With the recent trend toward internal combustion engines with lighterweight, higher outputs and higher rotating speed, development of pistonrings with smaller width has been positively made. The reduction in thewidth of the piston ring makes it possible to reduce the weight of thepiston ring, stabilize the behavior of the piston ring in a piston ringgroove and to decrease the thickness of the oil film, thereby improvingthe lubricating oil consumption.

However, the development of the piston rings with smaller width isaccompanied by a reduction in the oil film thickness, an increase ofwear of the rings and a shortening of the service life of the rings.Therefore, it has become impossible to use rings made of cast iron,which have hitherto been generally used, or rings made from carbonsteel, silicon-chromium steel or oil-tempered wire. Namely, rings madeof cast iron have the drawback that it is difficult to produce ringssmaller in size in the axial direction and the breaking strength thereofis unsatisfactory. The silicon-chromium rings are formed relativelylarge in cross-sectional area, in view of the poor high-temperaturestrength of silicon-chromium steel, and have a great inertia, which willbring about the fluttering phenomenon. Therefore, tool steels, springsteels and stainless steels have recently come to be used as materialsfor piston rings. Of stainless steels, particularly, 13Cr martensiticstainless steel (0.65C-13.5Cr-0.3Mo-0.1V) has been used for compressionrings to give good results. On the other hand, the oil ring has theimportant function of appropriately controlling the amount of thelubricating oil at the time of sliding contact between the piston ringsand the cylinder and scraping off excess lubricating oil to prevent itfrom penetrating into the combustion chamber. Therefore, side rails forthe oil ring are, like the compression rings, required to have heatresistance and wear resistance. The same material as that for thecompression rings has been used for the side rails to give good results.

The piston rings made of the martensitic stainless steel, however, arenot satisfactory in wear resistance and scuffing resistance when usedfor engines in which severe abrasive wear takes place. Compression ringsmade from martensitic stainless steel and subjected to a gas nitridingtreatment are unsatisfactory in strength of fitting to the piston, andhas the problem that they may be broken when the joint gap isexcessively widened (to 10T or above, effective durability being 11-13T,where T is thickness (mm) of ring). Further, such compression rings havethe drawback of being unsatisfactory in scuffing resistance and,therefore, being scuffed when used for internal combustion engines inwhich scuffing resistance requirements are severe. Accordingly, a thinNi--P or Ni--Co--P plating or such base plating with hard particles(e.g., Si₃ N₄) dispersed therein has been provided only on a slidingsurface of the compression ring. In view of the above, in connectionwith the piston rings made of martensitic stainless steel there has beena demand for further higher wear resistance and scuffing resistance inorder to prolong the service life of the piston rings.

In the internal combustion engines, rocker arms are operated in abutmentwith a cam shafts. As the cam shafts rotate in high rotational speed,the rocker arms are required to be resistant to wear and scuffing.

Further, other shafts used in automobiles and operated under severesliding condition with heavy load, such as pinion shafts of pinion gearsused in differential gear device for front-engine front-wheel-drivevehicles, are required to be resistant in seizure and to wear.

SUMMARY OF THE INVENTION

It is primary object of the present invention to provide a martensiticstainless steel having good wear resistance.

It is an object of the present invention to provide a martensiticstainless steel for use as a material for piston rings and rocker arms,and pinion shafts for differential gears, all for parts of automobiles,which has good wear resistance.

It is another object of the present invention to provide a wrought steelfor use as a material for piston rings, rocker arms, pinion shafts andso on which has good wear resistance, fitting strength and fatiguestrength and is particularly suitable for achieving enhancement of theoutput and rotating speed of internal combustion engines.

It should be understood that the term "piston rings" used in the presentinvention includes the meanings of compression springs, oil rings andside rails assembled to the oil ring.

The steel according to the present invention consists essentially of, byweight, 0.55-1.10% of C, up to 2.0% of Si, up to 2.0% of Mn, 12-25% ofCr and the balance of Fe and inevitable impurities.

The present inventors have made intensive studies of the wear resistanceof conventional martensitic stainless steels, with an idea that anincrease in the amount of chromium carbide will be effective inimproving the wear resistance of the steels. As a result of the studies,the present inventors have found out optimum ranges of contents of C,Si, Mn, etc. in connection with the content of Cr in the martensiticstainless steel.

According to the present invention, chromium carbide is formed in thesteel in a larger amount than in conventional steels by increasing theCr content, to thereby improve the wear resistance of the steel.Besides, C is added to the steel in an amount sufficient to formcarbides, and the upper limits of the C content as well as Si and Mncontents are set in such ranges as not to spoil cold workability of thesteel. Though the steel according to the present invention displayssufficient wear resistance when used as it is, it is preferable to usethe steel after a heat treatment, namely, a quenching and temperingtreatment or a quenching treatment (in the latter case, a subsequentnitriding treatment serves also as a tempering treatment). The effect ofimproving the wear resistance can be further enhanced greatly when thesteel thus heat treated is subjected to a surface treatment such asnitriding, plating, thermal spraying, etc. The surface treatment isapplied to a surface including a sliding surface, and may be any of anitriding treatment such as gas nitriding, gas soft nitriding, ionnitriding, salt bath nitriding, etc., plating such as Cr plating,composite plating, etc., coating with ceramic such as TiN, TiCxNy' TiC,etc. by physical vapor deposition (PVD) or chemical vapor deposition(CVD), metal spraying, etc.

According to the present invention, further, 0.2-2.0% of Ni may be addedto the steel, if required, to enhance high-temperature strength,hardenability and corrosion resistance, and at least one of 0.2-3.0% ofMo, 0.1-1.5% of V and 0.05-0.70% of Nb may be added to the steel, ifrequired, to refine the carbide particles and further improve the wearresistance.

Moreover, the present inventors have found that an addition of Al to theabove-mentioned martensitic stainless steel makes it possible toremarkably improve the wear resistance without lowering in hot and coldworkability at the stages of production of wires from an ingot of thesteel. The present inventors have then found that an addition of0.05-1.10% of Al, in connection with the contents of C, Si, Mn, Cr andthe like, enhances markedly the wear resistance and scuffing resistanceof the steel and lessens the wear of the opponent member. When theaddition of Al is accompanied by an addition of 0.2-2.0% of Cu, asrequired, it is possible to enhance corrosion resistance and oxidationresistance of the steel.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features and attendant advantages of the presentinvention will be more fully appreciated as the same becomes betterunderstood from the following detailed description when considered inconnection with the accompanying drawings, in which:

FIG. 1 is a micrograph (400 magnifications) showing the metallicstructure of a steel according to the present invention;

FIG. 2 is a micrograph (400 magnifications) showing the matallicstructure of another steel according to the present invention;

FIG. 3 is a diagram showing the relationship between carbide graindiameter and area ratio, of steels according to the present invention;

FIG. 4 is a perspective view of a compression spring;

FIG. 5 is a vertical cross-sectional view of a three pieces combinationtype oil ring in a sliding condition.

FIG. 6 is a diagram showing abrasion losses of the steel according tothe present invention and conventional steels; and

FIG. 7 is a diagram showing seizure loads of the steel according to thepresent invention and conventional steels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a steel having good wear resistancewhich consists essentially of, by weight, 0.55-1.10% of C, up to 2.0% ofSi, up to 2.0% of Mn, 12.0-25.0% of Cr and the balance of Fe andinevitable impurities.

The steel according to the present invention can be improved incorrosion resistance, toughness and hardenability by incorporating0.20-2.0% of Ni into the steel in addition to the above-mentionedchemical components, or can be improved in high-temperature strength andsurface hardness by the addition of at least one of 0.2-3.0% of Mo,0.1-1.5% of V and 0.05-0.70% of Nb to the steel.

The steel according to the present invention can be enhanced in wearresistance and scuffing resistance by incorporating 0.05-1.10% of Alinto the steel in addition to the above-mentioned chemical components.When the addition of Al is accompanied by an addition of 0.2-2.0% of Cu,it is possible to enhance corrosion resistance and oxidation resistanceof the steel.

The properties of the chemical components of the steel according to thepresent invention and the reasons for the limitations of the contents ofthe components will now be explained below.

C; 0.5-1.10%

Carbon is an element for obtaining a desired hardness on quenching andfor forming carbides to provide high strength and wear resistance. Ifthe C content is less than 0.55%, the amount of the carbides formed aresmall, and the wear resistance obtained by the presence of the carbidesis poor. If the C content exceeds 1.10%, on the other hand, the particlesize of the carbides is increased, resulting in abrasion of the cylinderliner (opponent member of the piston ring), and it is impossible toshape the steel into the piston ring by cold working. Therefore, theupper limit of the C content is set at 1.10%. In order to obtain anoptimum hardness (Hv 350 to 450), a C content at least 0.80% ispreferred.

Si; 0.10-2.0%

Silicon is added at the time of refining as a deoxidizing agent, andserves to provide heat resistance. An addition of at least 0.10% of Siis necessary for obtaining the effects. However, an addition of a largeamount of Si impairs cold workability such as drawability of the steel.Thus, the upper limit of the Si content is set at 2.0%.

Mn; 0.10-2.0%

Manganese is added at the time of refining as a deoxidizing agent, justlike Si, and serves to increase toughness. An addition of at least 0.10%of Mn is required for obtaining these effects. However, an addition of alarge amount of Mn impairs cold workability of the steel. Therefore, theupper limit of the Mn content is set at 2.0%.

Cr; 12.0-25.0%

Chromium combines with carbon to form a carbide, thereby enhancing wearresistance, and serves also to enhance corrosion resistance and matrixstrength and to increase the hardness of the nitrided hardened case. Ifthe Cr content is less than 12.0%, the effects of the Cr addition,particularly the enhancement of wear resistance, cannot be displayedsatisfactorily. Thus, the lower limit of the Cr content is 12.0%.However, if Cr is added in a large amount, the effects are notconspicuously displayed and toughness is lowered, resulting in poor coldworkability. Therefore, the upper limit of the Cr content is set at25.0%. The Cr content may be 19.5-25.0% in the case where Al is notadded to the steel.

Ni; 0.2-2.0%

Nickel serves to provide corrosion resistance, toughness andhardenability. Particularly in the case of the addition of Al, which hasa high tendency to form ferrite, nickel serves to prevent formation offerrite at high temperatures and improve hardenability and hotworkability. Since these effects are low if the Ni content is less than0.2%, the lower limit of the Ni content is 0.2%. On the other hand, a Nicontent of more than 2.0% impairs cold workability of the steel. Thus,the upper limit of the Ni content is set at 2.0%.

Mo; 0.2-3.0%

Molybdenum, like Cr, forms a carbide to enhance the hardness of thenitrided case upon nitriding, thereby enhancing wear resistance, andserves also to increase high-temperature strength of the steel. Toobtain these effects, a Mo content of at least 0.2% is required.However, when the Mo content is more than 3.0%, the effects becomeinconspicuous and hot workability is degraded. Therefore, the upperlimit of the Mo content is set at 3.0%.

V; 0.10-1.50%, Nb; 0.05-0.70%

Vanadium and niobium serve to increase resistance to softening ontempering and high-temperature strength as well as to refine carbides,and they form nitrides on nitriding, thereby enhancing the casehardness. To obtain these effects, at least 0.10% of V or at least 0.05%of Nb is required. However, if more than 1.5% of V or more than 0.70% ofNb is contained in the steel, coarse eutectic carbides are formed todeteriorate hot workability. Therefore, the upper limits of V and Nbcontents are set at 1.5% and 0.70%, respectively.

Al; 0.05-1.10%

Aluminum is dissolved into the matrix in a quenching and temperingprocess, and, through grain refining, it increases the strength of thematrix, thereby enhancing wear resistance and scuffing resistance. Asmentioned above, Al enhances wear resistance and scuffing resistancethrough spheroidizing the carbides and uniformizing the particle size,upon the quenching and tempering process. Besides, the Al dissolved inthe matrix precipitates as fine AlN on gas nitrization to remarkablyincrease the hardness of the nitrided layer, thereby markedly enhancingthe wear resistance and scuffing resistance and preventing increase ofthe abrasion of the cylinder bore. Further, the strength displayed atthe time of a joint gap enlarging test is remarkably enhanced. To obtainthese effects, it is necessary to add at least 0.05% of Al, and theeffects are increased as the amount of Al added increases. An Al contentof more than 1.10% renders the effects inconspicuous and causesformation of inclusions of Al₂ O₃, resulting in deterioration of surfaceproperties on hot and cold rolling (marked reduction in hardness whensurface flaws remain), reductions in strength and heat resistance, andembrittlement of the nitrided case. Therefore, the upper limit of the Alcontent is set at 1.10%.

Cu; 0.2-2.0%

Copper serves to enhance corrosion resistance and oxidation resistanceof the steel, and strengthens the matrix. To obtain these effects, anaddition of at least 0.2% of Cu is necessary. However, if more than 2.0%of Cu is added, hot workability is impaired and resistance to nitridingis increased. Thus, the upper limit of Cu content is set at 2.0%. The Cucontent may be 0.2-1.0% where Al is not added to the steel.

Table 1 shows the chemical compositions of steels according to thepresent invention and a comparative steel served to an abrasion test anda seizure test. In the table, steels A01 to A34 are steels according tothe present invention, and steel B1 is 13Cr martensitic stainless steelconventionally used for piston rings, prepared by way of comparison.

Each of the steels to be served to abrasion test and seizure or gallingor scaring test was melted in an electric furnace, cast, hot rolled, andthen quenched and tempered (target hardness Hv350-450). From the thustreated steels, 10×15.7×6.3 mm test pieces for abrasion test and 30×30×5mm test pieces for seizure test were prepared. The test pieces forabrasion test thus obtained were served to the abrasion test under thefollowing conditions.

Abrasion test on LFW-1 abrasion tester

Opponent member: FC25 (Japanese Industrial Standard)

Load: 60 kg

Time: 120 min

Speed: 0.3 m/sec

Lubricating oil: low-viscosity engine oil (supplied 1.5 cc/min)

The abrasion loss at the sliding surface of each test piece tested wasmeasured. The results are shown in Table 2.

                                      TABLE 1                                     __________________________________________________________________________    Chemical composition (wt %)                                                   Test steel                                                                          C  Si Mn Ni Cr Mo V  Nb Al Cu                                           __________________________________________________________________________    A01   0.62                                                                             0.31                                                                             0.45  19.9                                                        A02   0.68                                                                             0.48                                                                             0.42  22.0                                                        A03   1.05                                                                             0.28                                                                             0.32  23.5                                                        A04   0.85                                                                             0.43                                                                             0.44                                                                             0.95                                                                             19.9                                                        A05   0.96                                                                             0.35                                                                             0.47                                                                             1.15                                                                             22.0                                                        A06   1.05                                                                             0.39                                                                             0.34                                                                             1.05                                                                             24.5                                                        A07   0.63                                                                             0.32                                                                             0.45  20.0                                                                             2.54                                                     A08   0.65                                                                             0.36                                                                             0.47  22.1                                                                             1.05  0.36                                               A09   0.62                                                                             0.34                                                                             0.41  24.3                                                                             0.35                                                                             0.32                                                  A10   1.05                                                                             0.28                                                                             0.38                                                                             1.02                                                                             19.8                                                                             2.54                                                     A11   1.00                                                                             0.45                                                                             0.32                                                                             1.58                                                                             23.5                                                                             1.08  0.39                                               A12   1.05                                                                             0.23                                                                             0.48                                                                             1.82                                                                             24.5                                                                             2.58                                                                             0.34                                                  A13   0.88                                                                             0.43                                                                             0.45  20.8                                                        A14   0.96                                                                             0.33                                                                             0.41  21.7                                                        A15   0.87                                                                             0.31                                                                             0.41  21.3  0.72                                                  A16   0.94                                                                             0.52                                                                             0.36  22.3                                                                             1.01                                                                             0.35                                                                             0.25                                               A17   1.06                                                                             0.38                                                                             0.38  22.3                                                                             1.01                                                                             0.35                                                                             0.25                                               A18   0.89                                                                             0.42                                                                             0.52                                                                             0.89                                                                             21.2                                                                             0.95                                                                             0.55                                                  A19   0.95                                                                             0.37                                                                             0.37                                                                             1.31                                                                             22.0                                                                             1.13                                                                             0.45                                                                             0.29                                               A20   0.61                                                                             0.30                                                                             0.44  19.8        1.00                                            A21   0.91                                                                             0.32                                                                             0.42  21.2        0.31                                            A22   0.95                                                                             0.29                                                                             0.35  13.7        0.95                                            A23   1.05                                                                             0.28                                                                             0.36                                                                             1.52                                                                             24.5                                                                             2.48                                                                             0.32  0.30                                            A24   0.95                                                                             0.30                                                                             0.32                                                                             0.98                                                                             21.2                                                                             0.32                                                                             0.10                                                                             0.29                                                                             0.31                                            A25   0.92                                                                             0.34                                                                             0.30                                                                             0.48                                                                             19.8                                                                             0.12                                                                             0.08                                                                             0.31                                                                             0.50                                                                             0.52                                         A26   0.91                                                                             0.32                                                                             0.34                                                                             0.72                                                                             21.1                                                                             0.28     0.34                                            A27   0.63                                                                             0.35                                                                             0.47                                                                             1.43                                                                             13.8        0.85                                            A28   0.88                                                                             0.46                                                                             0.53  17.2                                                                             0.46     0.41                                            A29   0.83                                                                             0.42                                                                             0.65  19.4  0.38  0.35                                            A30   1.02                                                                             0.54                                                                             0.46  18.3     0.38                                                                             0.55                                            A31   0.69                                                                             0.39                                                                             0.55                                                                             0.87                                                                             15.5                                                                             1.24     0.59                                                                             0.35                                         A32   0.75                                                                             0.43                                                                             0.46                                                                             0.45                                                                             20.1  0.26  0.43                                            A33   0.68                                                                             0.40                                                                             0.38  17.1  0.13                                                                             0.15                                                                             0.26                                            A34   0.84                                                                             0.33                                                                             0.53  21.7     0.24                                                                             0.53                                                                             1.25                                         B1    0.63                                                                             0.38                                                                             0.39  13.0                                                                             0.35                                                     __________________________________________________________________________

Then, other test pieces for abrasion test were subjected to gasnitriding by heating in an ammonium gas stream at 530° to 590° C. for atleast 5 hours. After the gas nitriding, the surface hardness of the testpieces was measured to be at least Hv 1000. The gas-nitrided test piecesfor abrasion test were served to the abrasion test under the sameconditions as above. The abrasion loss at the sliding surface of eachtest piece tested was measured, the results being also shown in Table 2.

Subsequently, the test pieces for seizure test were served to theseizure test under the following conditions, the seizure load measuredbeing also shown in Table 2.

Seizure test on mechanical testing laboratory type frictional abrasiontester

Opponent member: FC25 (Japanese Industrial Standard)

Load: Incremented by 25 kg at 2-min interval until seizure occurs.

Speed: 1.2 m/sec

Lubricating oil: Dropwise lubrication with low-viscosity engine oil

Seizure load: The load causing a sharp increase of frictionalcoefficient to or above 0.2 is taken as seizure load.

Of the comparative steels shown in Table 2, specimen B2 is one obtainedby applying hard chromium plating to the surface of the test piece ofsteel B1; the hard chrominum plating was tested for abrasion depth andseizure load.

                                      TABLE 2                                     __________________________________________________________________________    Abrasion test         Seizure test                                            (depth of abrasion, μm)                                                                          (seizure load, kg)                                                                             Enlarged amount                                                                         Fatigue                                 Surface          Surface    of joint gap                                                                            strength                                                                            Hardness                    Quenched                                                                            treated    Quenched                                                                            treated    causing breakage                                                                        gas-nitrided                                                                        after                  Specimen                                                                           and   by gas-                                                                            Plated                                                                              and   by gas-                                                                             Plated                                                                             of gas-nitrided                                                                         specimen                                                                            nitriding              (steels)                                                                           Tempered                                                                            nitriding                                                                          with Cr                                                                             Tempered                                                                            nitriding                                                                           with Cr                                                                            ring (mm) 50 kg/mm.sup.2)                                                                     Hmv (100               __________________________________________________________________________                                                           g)                     A01  3.40  2.20       125.0 150.0      ≧20 T                           A02  2.70  1.80       125.0 150.0      ≧20 T                           A03  2.20  1.50       137.5 167.5      ≧20 T                           A04  3.20  1.70       137.5 167.5      ≧20 T                                                                            7 × 10.sup.5           A05  2.40  1.30       137.5 167.5      ≧20 T                                                                            5 × 10.sup.5           A06  1.80  0.80       150.0 187.5      ≧20 T                                                                            5 × 10.sup.5           A07  3.30  2.00       137.5 167.5      ≧20 T                           A08  2.50  1.60       137.5 167.5      ≧20 T                           A09  1.90  0.90       137.5 175.0      ≧ 20 T                          A10  3.10  1.60       150.0 187.5      ≧20 T                                                                            7 × 10.sup.5           A11  1.90  0.90       150.0 187.5      ≧20 T                                                                            5 × 10.sup.5           A12  1.70  0.70       150.0 187.5      ≧20 T                                                                            5 × 10.sup.5           A13  2.50  1.70       125.0 150.0      ≧20 T    1345                   A14  2.40  1.60       137.5 167.5      ≧20 T    1337                   A15  2.30  1.30       137.5 167.5      ≧20T     1364                   A16  2.30  1.10       137.5 167.5      ≧20 T    1348                   A17  2.20  0.90       150.0 175.0      ≧20 T    1356                   A18  2.20  1.10       150.0 187.5      ≧20 T                                                                            7 × 10.sup.5                                                                  1332                   A19  2.10  0.80       150.0 187.5      ≧20 T                                                                            7 × 10.sup.5                                                                  1337                   A20  2.50  1.10       150.0 187.5      ≧20 T    1450                   A21  1.80  0.90       137.5 175.0      ≧20 T    1335                   A22  4.00  2.00       137.5 167.5      ≧20 T    1425                   A23  1.20  0.20       150.0 187.5      ≧20 T    1345                   A24  1.60  0.75       137.5 167.5      ≧20 T    1337                   A25  2.40  0.90       137.5 167.5      ≧20 T                                                                            2 × 10.sup.6                                                                  1364                   A26  1.70  0.80       137.5 167.5      ≧20 T                                                                            3 × 10.sup.                                                                   1348                   A27  2.90  1.00       137.5 167.5      ≧20 T    1356                   A28  1.90  0.80       137.5 167.5      ≧20 T    1332                   A29  1.70  0.55       137.5 175.0      ≧20 T    1377                   A30  1.60  0.75       150.0 187.5      ≧20 T    1378                   A31  2.50  1.20       137.5 167.5      ≧20 T    1332                   A32  1.80  0.95       150.0 175.0      ≧20 T    1364                   A33  2.00  1.00       137.5 167.5      ≧20 T    1305                   A34  1.50  0.60       150.0 187.5      ≧20 T    1364                   B1   5.80  3.50       100.0 137.5      11˜13 T                                                                           2 × 10.sup.5                                                                  1180                   B2              10.0              150                                         __________________________________________________________________________

As seen from Table 2, the abrasion loss of steel B1 according to theprior art was 5.8 μm for the quenched-and-tempered specimen, 3.5 μm forthe nitrided specimen, and 10.0 μm for the Cr-plated specimen. On theother hand, the abrasion losses of steels A01 to A19 according to thepresent invention were 1.7-3.5 μm for the quenched-and-temperedspecimens and 0.7-2.2 μm for the nitrided specimens (the hardness of thenitrided case was over Hv 1000 in all cases), the values confirmingexcellent wear resistance. The reason for the superior wear resistanceof the steels according to the present invention, with or without thenitriding treatment, is considered to be the formation of larger amountsof chromium carbides (slightly above 2.0 μm in average particle size),as compared with steel B1 according to the prior art, due to thecompositions of 0.55-1.10% C and 12.0-25% Cr. Addition of Mo, V and Nbleads to formation of fine carbides, so that wear resistance is enhancedmore as the amounts of these elements are larger.

Besides, the carbide-forming elements Cr, Mo, V and Nb areferrite-forming elements, and addition of large amounts of theseelements causes, depending on the C and Ni contents, precipitation of αphase (ferrite) (steels A01, A02, A07, A08 and A09 according to thepresent invention), resulting in inferior wear resistance as compared tothose of uniform martensite structures free of precipitation of α phase(steels A03 to A06 and A10 to A12 according to the present invention).Accordingly, the addition of Ni is important for obtaining excellentwear resistance, in the point of strengthening the matrix whileprecipitating large amounts of carbides.

The enhancement of wear resistance by the gas nitriding treatment, tothe level by far superior to the wear resistance ofquenched-and-tempered specimens, is attributable to precipitationhardening (Hv 1000 or above) by precipitation of fine chromium nitridein the matrix, conversion of chromium carbide into chromium carbonitrideor chromium nitride, and wavy precipitates (considered to be grainboundary cementite) formed from carbon excluded from carbides.

In addition, steels A20 to A34 according to the present invention showedan abrasion depth for quenched-and-tempered specimens of 1.20-4.00 μm,as contrasted to the value of 5.8 μm attained with steel B1 according tothe prior art. The gas-nitrided specimens of the steels of the presentinvention showed an abrasion depth of 0.20-2.00 μm, as contrasted to 3.5μm of steel B1. Thus, in both cases, the results confirmed the excellentwear resistance of the steels according to the present invention.

The results of the seizure test will now be discussed.

For piston rings, a good scuffing resistance of a seizure load of atleast 125.0 kg is sufficient to produce good results, without generationof scuffing, in actual engine operations. On the other hand, when amaterial having a seizure load of less than 112.5 kg is used, top ringsare scuffed and fatally damaged under such severe engine operationconditions that an oil film is partly broken. Oil ring and side railsare lightly marred vertically, if not so heavily as the top rings.Therefore, a material having a higher seizure load can be used in athermally severer engine operation condition.

The nitrided specimens of the steels according to the present inventionshowed a scuffing resistance comparable or superior to that of steel B1according to the prior art. Particularly, steels A03 to A05, A07 and A08of the present invention showed a scuffing resistance of 167.5 kg, andsteels A06 and A10 to A12 of the invention showed a superior scuffingresistance of 187.5 kg. The reason why these steels particularly showthe excellent scuffing resistance is that granular chromium carbonitrideor chromium nitride which scarcely adheres to the opponent memberprojects slightly from the matrix at the sliding surface to preventadhesion of the matrix to the opponent member, and traces of adhesion,if present between the opponent member and the matrix, are cut off bythe granular chromium carbonitride or chromium nitride, therebypreventing occurrence of heavy seizure.

The seizure loads of steels A20 to A34 were 137.5 to 150.0 kg forquenched-and-tempered specimens and 167.5 to 187.5 kg for gas-nitridedspecimens, as contrasted respectively to 100.0 kg and 137.5 kg of thespecimens of the conventional steels. In both cases ofquenched-and-tempered specimens and the gas-nitrided specimens, it wasconfirmed that the steels according to the present invention haveexcellent scuffing resistance.

In fitting a piston ring into the ring groove of a piston, the joint gapof the piston ring having a given radial thickness (T size, mm) isenlarged to 10 times the radial thickness (hereinafter referred tosimply as 10T). Therefore, the piston ring must at least have a fittingstrength of more than 10T. Though piston rings formed of thequenched-and-tempered specimens show sufficient fitting strength, pistonrings formed of the nitrided specimen of steel B1 according to the priorart show a marginal fitting strength of 11 to 13T because of the brittlediffusion-hardened layer, and may be broken under some variations in thematerial and the enlarging amount of the joint gap. In the case of thesteels of the present invention, on the other hand, piston rings for abore diameter of 86 mm (B size 2.0 mm, T size 3.15 mm, and nitridingdepth 90 μm) have, as shown in Table 2, a superior fitting strength ofat least 20T, as contrasted to 11-13T of steel B1 of the prior art.

The hardnesses of gas-nitrided specimens are also shown in Table 2.While steel B1 according to the prior art showed a hardness of 1180,steels A20 to A34 of the present invention had a hardness of 1305 to1450, confirming the high hardness of the nitrided case of the steels ofthe present invention.

Piston rings to which fatigue strength matters, such as keystone typerings, are subjected to breakage, particularly where a brittle materialsuch as a gas-nitrided material is used and where a brittle compositeplating is applied to a surface area for sliding contact with thecylinder wall. Gas-nitrided piston rings were served to a fatigue testin a diluted aqueous solution of sulfuric acid under an amplitude stressof 50 kgf/mm. The results are shown in Table 2. While steel B1 of theprior art showed a fatigue strength of 2×10⁵, steels A05, A06, all andA12 of the present invention showed a fatigue strength of 5×10⁵, steelsA04 and A10 showed 7×10⁵, steel A16 showed 1×10⁶, steels A17 and A18showed 2×10⁶, and steel A19 showed 3×10⁶, indicating a markedimprovement in fatigue strength. The excellent fitting strength andfatigue strength are attributable to the strengthening of the matrix ofthe gas-nitrided diffusion-hardened case by the addition of 12.0-25.0%of Cr.

Top rings formed from steel A21 according to the present invention andtop rings formed from steel A14 according to the present invention(different from steel A21 only in that steel A14 do not contain A1) wereindividually fitted to pistons for a 2000-cc in-line four-cylinderengine, and a 150-hr endurance test was carried out. Upon this test, theabrasion loss of the cylinder bore used with the top ring of steel A14was taken as 1.00, and the abrasion loss of the cylinder bore used withthe top ring of steel A21 was represented in terms of its ratio to theformer abrasion loss. The result is shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                         Steel A21                                                                             Steel A14                                            ______________________________________                                        Cylinder bore abrasion ratio                                                                     0.80      1.00                                             ______________________________________                                    

As seen from Table 3, the piston rings formed of the steel A21containing A1 produced less abrasion loss of the cylinder bore, ascompared to the piston rings of the steel A14 containing no A1. This isdue to spheroidization of carbide particles and uniformization ofcarbide particle diameter (reduction of the amount of coarse carbideparticles) upon the quenching and tempering process, increase in thehardness of the nitrided case by precipitation of fine particles of A1Nupon gas nitriding, or the like.

The reasons for the test results shown above will now be explained indetail below, based on FIGS. 1 to 5 and Table 4.

FIG. 1 is a micrograph (×400) showing the metallic structure of steelA21 according to the present invention, FIG. 2 is a micrograph (×400)showing the metallic structure of steel A14, FIG. 3 is diagram showingthe relationship between carbide particle diameter, at least 2 μm, andarea ratio, for the steels according to the invention, and Table 4 showsa comparison between the steels A21 and A14 according to the presentinvention in respect of average particle diameter of carbides and arearatio (the proportion of area of carbide particles present in the fieldwhen observed under a microscope).

                  TABLE 4                                                         ______________________________________                                                        Steel A21                                                                             Steel A14                                             ______________________________________                                        Average particle diameter                                                                       3.9       3.4                                               (μm)                                                                       Area ratio of carbide                                                                           0.54      0.98                                              particles at least 8 μm                                                    in diameter(%)                                                                ______________________________________                                    

As clearly seen from FIG. 1 and Tables 3 and 4, in steel A21 containingA1 in accordance with the present invention, the chromium carbideparticles are somewhat rounded in shape in compared with those in thesteel A14 which contains no A1, and the area ratio of coarse carbideparticles of the steel A21, 0.54%, is as low as 0.58% of the steel A14,so that cylinder bore abrasion in the steel A21 can be decreased to 4/5in compared with that in the steel A14.

The addition of A1, as distinguished from the addition of othercarbide-forming elements (C, Cr, Mo, V, Nb, W), has the advantageousfeatures that a desired remarkable improvement in wear resistance andscuffing resistance can be achieved by the addition of a small amount ofA1, and the A1 addition prevents increase of cylinder bore abrasion anddoes not cause substantial reduction in hot workability.

The above-mentioned effects obtained with the steels according to thepresent invention will be explained more in detail below, in connectionwith the application of the steels to piston rings.

(1) Top ring (first compression ring)

FIG. 4 shows a perspective view of a top ring 10. Of piston rings, thetop ring is most severely required to have good scuffing resistance, therequired value varying widely depending on the engine in which the topring is to be used. A top ring formed of the conventional 13Crmartensitic stainless steel is susceptible to scuffing if used withoutgas nitriding treatment. Therefore, the top ring of 13Cr martensiticstainless steel has been used after gas-nitriding the top ring orsurface-treating the top ring only in a surface area for sliding contactwith the cylinder bore by hard chromium plating, thermal spraying, Ni-Pbased composite plating or the like.

The top ring formed of the steel according to the present invention,even as-quenched-and-tempered, shows a scuffing resistance comparable tothat of the gas-nitrided product of 13Cr martensitic stainless steeland, therefore, can be satisfactorily used as it is. Even in engineswith severer requirements for scuffing resistance, the steel of thepresent invention, when surface treated, gives a scuffing resistancesuperior to that of the hard chromium plated conventional steel, andproduces good results without generation of scuffing.

As for wear resistance, also, the conventional 13Cr martensiticstainless steel is not necessarily satisfactory, and it has been acommon practice to adopt a large nitriding depth of 90 or 120 μm for thetop ring to be used in engines with severe requirements. However, anincrease in the nitriding depth leads to a lowering in fatigue strengthand fitting strength at the joint gap 12 of the ring 10, and maytherefore cause breakage of the ring.

On the other hand, the steel according to the present invention has animproved strength and, even with the same nitriding depth as in theprior art, is free from the ring breakage problem. In addition, becauseof the marked improvement of the wear resistance, the nitriding depthcan be decreased, leading to a further higher strength, a shorter gasnitriding time and an easier mass production of the top rings. Besides,the decrease of wear loss minimizes the deterioration in oil consumptionperformance and blow-by gas performance associated with wear, andprevents the deteriorations in total engine performance.

(ii) Oil ring

An oil ring is accompanied by a high contact surface pressure due totension and, in some engines, may wear more heavily than a top ring.Since a lowering in the contact surface pressure due to the wearincreases oil consumption, the wear resistance requirements for the oilrings are considerably severe. The steel according to the presentinvention has eminent wear resistance and is able to meet therequirement.

FIG. 5 is a cross-sectional view of a three pieces combination type ofoil ring 30 in a sliding condition, in which are shown an oil ringgroove of a piston, a cylinder bore 18, side rails 20 and anspacer-expander 22. For the side rails 20 of this type of oil ring 30,the depth of nitriding, if carried out, is at most 30-60 μm due to therestriction imposed for maintaining high strength. In an engine used fora long time, therefore, the scuffing resistance and wear resistance ofthe base steel (quenched and tempered steel) exposed due to wear-out ofthe nitrided case are important factors. The steel according to thepresent invention gives good results on this point, as mentioned abovein connection with the top ring.

In the case of the three pieces combination type oil ring 30, not onlythe surface area for sliding contact with the cylinder bore 18 but alsothe contact portions between the side rails 20 and ear portions of thespacer-expander 22 are required to have good wear resistance. Thisrequirement also is met by the steel according to the present invention.

Next, rocker arm pads were prepared using the steels A02, A05 and A10according to the present invention and comparative steel B3 (correspondsto SKD 11). These pads were subjected to quenching and temperingfollowed by subjected to salt bath nitriding operation at 530°-590° C.for 3 hours. Then rocker arm pads thus obtained were served to themotoring test under the following conditions.

Motoring test

Rotational speed of internal combustion engine: 2000 r.p.m.

Time: 200 hours

Valve spring load: More than 150% (in compared with fitting load of inmass-produced engine)

Lubricating oil: Deteriorated oil by long-term use

Cam shaft (opponent material): Cast iron alloy

The abrasion loss at the sliding surface of each of the rocker arm padand cam were measured.

The results are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        Test      Abrasion loss of                                                                             Abrasion loss of                                     steel     rocker arm pad (μm)                                                                       cam (μm)                                          ______________________________________                                        A02       3              5                                                    A05       2              6                                                    A10       1.5            8                                                    B3        6              80                                                   ______________________________________                                    

Table 6 shows a comparison between the steels A02, A05 and A10 accordingto the present invention and the comparative steel B3 in respect of arearatio of the carbide particles.

                  TABLE 6                                                         ______________________________________                                                Area ratio of carbide                                                                        Area ratio of carbide                                  Test    particles at least                                                                           particles at least                                     steels  2 μm in diameter (%)                                                                      8 μm in diameter (%)                                ______________________________________                                        A02     7.6            0.7                                                    A05     8.1            0.9                                                    A10     7.5            0.8                                                    B3      5.2            1.8                                                    ______________________________________                                    

As seen from Tables 5 and 6, the abrasion loss of the steels accordingto the present invention show excellent wear resistance in compared withthe abrasion loss of the conventional steel. The reason for theexcellent wear resistance of the steels according to the presentinvention is considered to be the enhancement of wear resistance due tothe formation of increased amount of carbides and the decrease ofattacking the opponent material due to the refining of the carbideparticles (decrease of coarse carbides).

Further, pinion shafts were prepared using the steel A05 according tothe present invention and conventional steels B4 (SCr 415) and B5 (SCM440). Pinion shafts made of steels A05 and B5 were subjected to gassoft-nitriding treatment after quenching and tempering. Pinion shaftmade of steel B4 was subjected to carburizing, quenching and temperingbut not nitriding. Then pinion shafts thus obtained were assembled inthe differential gear assembly of a front-engine front-wheel-drivevehicle. Test drive under the condition in which pinion gears movesrelatively and actively on the pinion shaft due to the differentialmovement was conducted with respect to each of the pinion shaft. Afterdrive test of 50.000 km, each of the pinion shaft was disassembled fromthe differential gear assembly and abrasion losses were measured. Theresults are shown in FIG. 6.

As seen from FIG. 6, while the abrasion losses of the pinion shafts madeof conventional steel B4 and B5 (gas soft-nitrided) are as large as 40μm and 25 μm, respectively, the abrasion loss of the pinion shaft madeof steel A05 according the present invention is 5 μm, which is as low as1/5-1/8 of those of the conventional steels.

Specimens made from the above-noted pinion shafts were served to theseizure test. Seizure tests were carried out by mechanical testinglaboratory type frictional abrasion tester under the condition as statedearlier. The seizure load measured are shown in FIG. 7. It is to benoted that seizure may not occur on the pinion shafts made of the steelswhich show the abrasion load of more than 250 kg (required level) inaccordance with this test even when the pinion shafts are subjected tobe operated under the driving condition of vehicles in which thedifferential movement may frequently occur. Accordingly, the resultsconfirms the excellent seizure resistance of the steels according to thepresent invention.

Table 7 shows a comparison between the steels A03, A05, A22 and A26according to the present invention and the conventional steels B4 and B5in respect of abrasion loss measured after the drive test and seizureload measured on the frictional abrasion test.

                  TABLE 7                                                         ______________________________________                                                  Abrasion loss                                                                          Seizure load                                                         (μm)  (kg)                                                       ______________________________________                                        A03         2.8        450                                                    A05         3.0        450                                                    A22         5.0        450                                                    A26         2.2        450                                                    B4          40.0       150                                                    B5          25.0       200                                                    ______________________________________                                    

As seen from Table 7, Abrasion loss of the steels according to thepresent invention show excellent wear resistance in compared with theabrasion loss of the conventional steels B4 (carburized) and B5 (gassoft-nitrided) and seizure load of the steels according to the presentinvention is excellent in compared with the seizure load of theconventional steels.

A pinion shaft which is made from a steel including, by weight,0.55-1.10% of C and 12.00-25.0% of Cr, having fine chromium carbideparticles of 2-12 μm in diameter being dispersed inquenched-and-tempered martensite structure at area ratio of 0.2-8.0%,having a nitride layer of more than 2 μm on the surface thereof and adiffusion layer of more than 20 μm under said nitride layer obtained bysoft-nitriding treatment shows excellent wear resistance and seizureresistance. The reason for the excellent wear resistance and seizureresistance of the steels according to the present invention isconsidered to be obtained by the existance of fine chromium carbideparticles of 2-12 μm in diameter formed by soft-nitriding treatment andby the existance of wavy precipitates (considered to be grain boundarycementite) formed from carbon excluded form carbides.

Though the steel of the present invention shows satisfactory wearresistance even when used as it is, the effect can be remarkablyaugmented by a surface treatment such as nitriding, plating and thermalspraying. It is preferable to heat-treat the steel of the inventionprior to nitriding. The heat treatment may be, for instance, a quenchingand tempering treatment (with the subsequent nitriding serving also astempering). The nitriding treatment, which is applied to a surface ofthe steel including the surface area to be brought into sliding contact,may be any of gas nitriding, gas soft-nitriding, salt bath nitriding,tufftriding and ion nitriding.

As has been detailed above, the steel according to the present inventioncontains an increased amount of Cr, for further enhancement of the wearresistance and service life of the conventional martensitic stainlesssteels, and shows formation of chromium carbide in a larger amount thanin the conventional steels and a remarkable enhancement of wearresistance achieved by spheroidizing of the carbide particles anduniformization of particle size. By restriction of C, Si and Mn contentsand addition of Ni, Mo, V or Nb, the steel according to the presentinvention shows further enhanced wear resistance, scuffing resistance,fitting resistance and fatigue resistance while retaining the goodhigh-temperature resistance, corrosion resistance and scuffingresistance of the conventional steels. When the steel of the presentinvention is used for piston rings, longer service life of the pistonrings is ensured. The above-mentioned effects is further augmented by asurface treatment such as nitriding, plating and thermal sprayingapplied to the steel according to the present invention.

Furthermore, the steel according to the present invention is based onthe addition of 0.05-1.10% of A1, the optimum A1 content range found inconnection with the contents of C, Si, Mn, Cr or the like, wherebycarbide particles are spheroidized and the particle size is uniformized,leading to higher wear resistance and scuffing resistance. By a surfacetreatment such as gas nitriding, A1 dissolved in the matrix isprecipitated as fine A1N particles, resulting in further enhancement ofwear resistance and scuffing resistance. Moreover, the steel accordingto the present invention has many other effects in remarkably increasingthe fitting strength of piston rings, showing high hardness, beingsuperior to the conventional steels in fatigue strength, and so on.

What is claimed is:
 1. A steel having good wear resistance consistingessentially of, by weight, 0.55-1.10% of C, 0.1-0.52% of Si, up to 2.0%of Mn and 20.8-25.0% of Cr, and no intentionally added the remainderbeing Fe and inevitable impurities.
 2. A steel according to claim 1,wherein said steel consists essentially of, by weight, 0.55-1.10% of C,0.1-0.52% of Si, 0.10-2.0% of Mn, 20.8-25.0% of Cr and 0.2-2.0% of Ni,the remainder being Fe and inevitable impurities.
 3. A steel accordingto claim 1, wherein said steel consists essentially of, by weight,0.55-1.10% of C, 0.1-0.52% of Si, 0.10-2.0% of Mn, 20.8-25.0% of Cr anda member or members selected from the group consisting of 0.2-3.0% ofMo, 0.1-1.5% of V and 0.05-0.70% of Nb, the remainder being Fe andinevitable impurities.
 4. A steel according to claim 1, wherein saidsteel consists essentially of, by weight, 0.55-1.10% of C, 0.1-0.52% ofSi, 0.10-2.0% of Mn, 20.8-25.0% of Cr, 0.2-2.0% of Ni and a member ormembers selected from the group consisting of 0.2-3.0% of Mo, 0.1-1.5%of V and 0.05-0.70% of Nb, the remainder being Fe and inevitableimpurities.
 5. A wrought steel having good wear resistance consistingessentially of, by weight, 0.55-1.10% of C, 0.1 to 0.52% of Si, 0.1 to2.0% of Mn, 20.8-25.0% of Cr and no intentionally added A1, theremainder being Fe and inevitable impurities, said steel having beensurface treated at least in a surface for sliding contact with anopponent member.
 6. A wrought steel according to claim 5, wherein saidsteel consists essentially of, by weight, 0.55-1.10% of C, 0.1-0.52% ofSi, 0.10-2.0% of Mn, 20.8-25.0% of Cr and 0.2-2.0% of Ni, the remainderbeing Fe and inevitable impurities.
 7. A wrought steel according toclaim 5, wherein said steel consists essentially of, by weight,0.55-1.10% of C, 0.1-0.52% of Si, 0.10-2.0% of Mn, 20.8-25.0% of Cr anda member or members selected from the group consisting of 0.2-3.0% ofMo, 0.1-1.5% of V and 0.05-0.70% of Nb, the remainder being Fe andinevitable impurities.
 8. A wrought steel according to claim 5, whereinsaid steel consists essentially of, by weight, 0.55-1.10% of C,0.1-0.52% of Si, 0.10-0.2% of Mn, 20.8-25.0% of Cr, 0.2-2.0% of Ni and amember or members selected from the group consisting of 0.2-3.0% of Mo,0.1-1.5% of V and 0.05-0.70% of Nb, the remainder being Fe andinevitable impurities.